Blender Cycles: Diffuse only lamp
I think it might be the node setup for the light. When I set up a similar scene with a pure emission shader on the plane, everything worked as I think you wanted.





In the scene I have a plane with a 50/50 blue diffuse/white gloss mixed shader, a 100% green diffuse shader and a 100% red glossy shader all at about .2-.3 roughness. There is also the default amount of light being emitted by the world

Finally just to prove that the red glossy plane isn't reflecting the light, I turned off the world light and you can see that the red plane is no longer being lit by any light source and is completely black.



Hope this helps you.